We seek to understand the molecular and cellular mechanisms that determine synaptic connectivity in developing nervous systems. Our work primarily focuses on motor axon guidance and synaptogenesis in the neuromuscular system of the Drosophila embryo. This system is attractive because it contains only 34 motoneurons and 30 muscle fibers, and the pattern of neuromuscular connections is genetically determined and essentially invariant between embryos. Most neural structures in mammals are much more complicated, and mammalian neurons are usually not individually specified. Nevertheless, many of the molecules and mechanisms involved in process outgrowth, axon guidance, and synaptogenesis are conserved between flies and humans. Thus, the results of our studies are likely to be relevant to an understanding of human neural development. We have concentrated on a group of cell surface/signal transduction molecules, the axonal receptor-linked protein-tyrosine phosphatases (RPTPs), and have shown that they control specific motor axon guidance decisions. Some of these RPTPs have mammalian counterparts with very similar structures (e.g., fly DLAR and human LAR), and RPTPs in vertebrates are also expressed in neuronal processes. We now wish to advance our understanding of the mechanisms involved in axon guidance by characterizing genetic and biochemical interactions among the RPTPs. We will focus on two RPTPs (DPTP99A and DLAR) that oppose each other's signaling pathways in which these and other RPTPs function. This will be done by examining interactions with the transmembrane proteins gp150 and Appl, which bind to and are substrates for fly RPTPs, and by conducting a biochemical a biochemical screen for new substrates using the recently developed 'substrate trap' method. We will also attempt to identify cell-surface ligands for RPTPs using expression cloning techniques. Finally, we will perform a genetic screen for new cell recognition/signaling molecules involved in axon guidance and synaptogenesis. This 'modular misexpression' P element screen is designed to identify genes for which over-expression in all muscles or all neurons produces axon guidance phenotypes. Some of the proteins encoded by which genes might function in RPTP signaling most are likely to be involved in different pathways and processes. We are especially interested in cell-surface proteins that control innervation of individual muscle fibers, a process that is still poorly understood.